This is but one of many types of access valves. Most are pretty easy to figure out, though, if you understand this one. You use an "Allen" wrench to screw this plunger down (closed) and unscrew it to open. Normally, you will not need to do any of this because screwing the hose onto the port on the side will give you the pressure reading you seek. When you remove the hex cap on the service valve, you find a female hex opening to allow the moving of the internal plunger that blocks the flow of the refrigerant inside the copper lines. The main purpose of the liquid line valve plunger is to pump-down the system and, of course, it is closed during shipping and storage. If you close (rotate clockwise) you can stop the flow of refrigerant at this valve plunger and pump all the refrigerant into the condenser section and thus be able to work on the rest of the system with no refrigerant in it. You do this procedure while running the system. Let it run for a bit to become stable and then close the liquid line valve plunger. The refrigerant can no longer get past the valves plunger and the compressor will suck the refrigerant from the rest of the system, pump it into the condenser and refrigerant will be stopped by the valve plunger on the other end of the condenser coils. Watch the gauge pressures and when they both get NEAR the zero pounds, stop the unit and the pressures should remain as you last saw it (while it was running). All the refrigerant is in the condenser. The plunger holds it at one side of the condenser coil and the discharge valves inside the compressor should hold the other side of the condenser pressure from coming back into the compressor. This will hold all the refrigerant in the condenser and will leave no pressure on the compressor crankcase, suction line, evaporator and the liquid line (from the cooling coil to the condenser liquid valve plunger that is closed). If the pressures begin to creep back up, then the liquid line valve plunger is leaking or not closed well. Or, the discharge valves inside the compressor may be cracked or broken. You can quickly close the plunger on the suction line (if the problem is the compressor valves) as soon as you turn off the unit from running and it will at least confine the leaking pressure to the compressor (and of course the condenser coils). This should explain the need (or convenience) for the existence of the service port plungers.
TIP:Three kinds of complications can occur when
attempting to pump-down a system.
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